- What is the technical term for this?
Bending downwards is called sagging. (Technically "sagging" isn't moving downwards; it's bending in a shape that looks like the mouth on a smiley face).
One thing you'll see with concrete structures is creep. This is where under constant load, the deflection increases over time.
- Is this a problem or not? Why?
Sagging deflection is inevitable. If you put a load on something it will move. Whether this movement is big enough to be visible or not it another question, but it will move.
When designing something we often check for two different failures: ultimate strength and deflection. (Note: There are plenty of other things we check for, this is a simplification.) Ultimate strength check = is it going to fall down. Deflection check = is it going to deflect so much that it scares people.
When assessing (i.e. checking) an existing structure, we generally only do ultimate strength checks; we don't worry so much about whether people are worried or not...
So, your structure has clearly failed the deflection limits, as it's scared you! But that's no indication as to whether or not it has exceeded its ultimate strength limits, so its no indication of whether there's actually a problem or not.
- Any actions should be done in order to prevent an accident?
Get a structural engineer to assess it. You've said in comments that the structure has been inspected due to this deformation - as long as that inspection has been carried out by a suitably experienced/qualified engineer, then you have no need to worry. I have no idea of how you satisfy yourself of their qualifications in your country.